The invention herein relates to a method for oxygenating blood and to a bubble-type oxygenator of the kind used in thoracic surgery, and more particulary to an oxygenator having improved bubble formation and improved oxygen-carbon dioxide exchange.
The history of safe and reliable blood oxygenators is relatively brief. Such oxygenators are used in open-heart surgery and other operations and treatments of the body when it is necessary to establish an extracorporeal circulation system for temporarily assuming the functions of the heart and lungs of the patient. In such a system, the oxygenator operates to perform the function usually performed by the lungs of the patient, i.e., the life-supporting transfer of oxygen into the blood and carbon dioxide out of the blood. The oxygenator is used in association with a pump which performs the function of the heart to cause circulation of the blood. Thus, early versions of the oxygenator were often referred to as "heart-lung" machines. The early heart-lung machines were typically rotating discs which passed through a pool of blood, but were only partially immersed therein such that the free surface of the disc exposed the blood to oxygen and accomplished some gas transfer. After this, bag-type oxygenators were introduced which were superior to the disc oxygenators, but which left much to be desired.
A major advance occurred in the mid-1960's when the rigid (or hard shell) bubble oxygenator was developed. The history of such oxygenators had its beginnings in the device shown in Raible, et al, U.S. Pat. No. 3,468,631, which is incorporated herein by reference, and they first came into clinical use with the development of the devices shown in Bentley et al, U.S. Pat. Nos. 3,488,158 and 3,578,411 which have come to be known as the Bentley Oxygenator. At the present time, such oxygenators are used more frequently than any other type. Among the important features of the oxygenators disclosed in the foregoing patents was the provision of a self-contained heat exchanger.
In the intervening years, some relatively minor modifications have been made in bubble oxygenators, e.g., those disclosed in Brumfield U.S. Pat. Nos. 3,764,271 and 3,769,162. However, all rigid bubble oxygenators shown in the aforesaid patents and all other such oxygenators known to applicant to have been put to actual clinical use have had one fundamental feature in common, namely, each of them introduced blood and oxygen-rich gas in the lower region of the device and caused a column of bubbles to flow upwardly through the initial portion of the device. Bentley, et al, U.S. Pat. Nos. 3,488,158 and 3,578,411 and the aforesaid Brumfield patents do have some downward portions in the flow path of the gas blood mixture, but it is clear that they were designed to provide for initial upward flow of the gas and blood mixture in that portion of the flow path where the bubbles are formed. In addition, Fields U.S. Pat. No. 3,204,631, discloses an oxygenator in which blood enters at an upper portion and oxygen enters at a lower portion such that there is a counterflow relationship with the blood initially flowing downwardly and the oxygen flowing upwardly.
The present invention is a further improvement of the device shown in the Bentley U.S. Pat. No. 3,615,238, issued Oct. 26, 1971, entitled "Oxygenator"; the Bentley, et al, U.S. Pat. No. 3,578,411 issued May 11, 1971, entitled "Bubbler Assembly for Blood Treating Apparatus"; the Bentley, et al U.S. Pat. No. 3,488,158 issued Jan. 6, 1970, entitled "Bubbler Assembly for Oxygenator"; and applications, Ser. Nos. 436,913, now abandoned, and 565,043, now U.S. Pat. No. 4,058,369 titled "Blood Oxygenator" and "An Improved Oxygenating Device", respectively, the disclosures of which are incorporated by reference herein. These devices each represent important developments in the blood treatment art. However, since these devices temporarily assume the function of the heart and lungs of a patient during certain operations or other treatments of the body, further improvements are desired which will effect within such devices a blood treatment process as equivalent as possible to that natural process effected by the heart and lungs.
One aspect of the human oxygenating process which has heretofore been difficult to duplicate concerns the ratio of oxygen in the blood to carbon dioxide commonly expressed as the physiological ratio of pO.sub.2 to pCO.sub.2. In the past, oxygenating devices were either unable to maintain this pO.sub.2 to pCO.sub.2 ratio or, in an effort to maintain such a ratio over the range of flow rates required during operation of the devices, have operated inefficiently and/or in a manner which may adversely affect the blood. For example, when an increase in pO.sub.2 was desired, it could be effected only by a substantial increase in the flow of oxygen with respect to the flow of blood into the device. A high gas-to-blood-flow ratio represented an inefficient operation of the oxygenator and, more importantly, substantially increased the risk of hemolysis.
Therefore, there is a need for a method and a device for oxygenating blood whereby (a) oxygen to blood transfer can be effectively and efficiently achieved with an improved gas-to-blood flow rate and (b) improved blood and blood bubble flow characteristics can be obtained.